Perhaps the simplest and most ancient of heating techniques is to expose the workpiece directly to a flame. Of the many techniques used by industry for generating a flame, one of the most common is a gas burner in which a jet of combustible gases exiting a burner head is ignited. The heat generated by the burner is controlled to some degree by controlling the flow rate of the gases, and the position and shape of the flame, by adjusting the configuration of the burner head.
It is sometimes necessary to quickly extinguish the flame generated by a gas burner to avoid setting the workpiece aflame or to allow workmen access to the burner head or workpiece to correct a malfunction.
Furthermore, gas burners sometimes "flashback," or propagate the flame backwards along the gas flow path into the burner apparatus itself, perhaps damaging the apparatus or causing an explosion capable of injuring workers nereby.
More specifically, fluids are driven to flow by fluid pressure. For example, the fuel in a burner is driven toward the burner head at a flow rate dependent in part on the pressure exerted by the fuel supply. Once past the burner head, the fuel is ignited and burns. If the amoutn of fuel reaching a flame position in a given time balances the amount of fuel consumed in that time, the flame remains stationary.
A drop in pressure slows down the fuel. Thus, if the flame is stationary before a pressure drop, the rate of combustion may exceed the flow rate after the pressure drop. The flame could then move upstream, "looking" for more fuel. Unless stopped or extinguished, this "flashbakc" will enter the burner and move upstream into the heating apparatus.
The danger of flashback is especially acute where the gas being supplied to the burner is a mixture of a combustible gas and oxygen. For example, if the gas being supplied is a stoichiometric mixture of hydrogen and oxygen from a gas generator which dissociates water into its component elements, the rate of combustion is high. As a result, a smaller drop in pressure will result in a more pronounced flashback.
One technique for preventing flashback is to use a one-way check valve, which permits the flow of gas only toward the burner head so as (theoretically) not to permit a combustion reaction to flow back into the apparatus. Unfortunately, a check valve may conduct enough heat into the apparatus, to allow the gas behind the valve the ignite. Even were the check valve to prevent flashback in all conceivable cases, it is a passive device which cannot extinguish the flame downstream of its position to allow access to the burner head or workpiece.